Adhesion by plasma conditioning of semiconductor chip surfaces

ABSTRACT

A plasma conditioning method of improving the adhesion between an integrated circuit chip, having active and passive surfaces, the active surface polymer-coated and having a plurality of electrical coupling members, and an insulating underfill material. The method comprises the steps of positioning a wafer having a plurality of integrated circuits, including the coupling members, in a vacuum chamber of a plasma apparatus so that the polymer-coated surface faces the plasma source. Next, a plasma is initiated; the ion mean free path is controlled so that the ions reach the wafer surface with predetermined energy. The wafer surface is then exposed to the plasma for a length of time sufficient to roughen the polymer surface, clean the polymer surface from organic contamination and improve the surface affinity to adhesion. The adhesion ability of this surface to organic underfill material is thus enhanced.

This application is a division of Ser. No. 11/047,519, filed Feb. 1,2005, which is a division of Ser. No. 09/952,454, field Sep. 14, 2001,now Pat. No. 6,869,831 from which priority is claimed under 35 U.S.C.120.

FIELD OF THE INVENTION

The present invention is related in general to the field of electronicsystems and semiconductor devices and more specifically to the field offlip-chip assembly with underfilling, in which strong adhesion isrequired between the underfilling material, the semiconductor chip andthe substrate.

DESCRIPTION OF THE RELATED ART

In flip-chip assembly, the active surface of a semiconductor chip,including the integrated circuits, is connected face-down to thesubstrate by coupling members, usually solder balls attached, on oneside, to the chip and, on the opposite side, to the substrate. Thesesolder balls thus create a gap between chip and substrate.

Important aspects of the flip-chip assembly of semiconductor chips havebeen studied in the 1960's by IBM researchers and published in a seriesof papers (IBM Journal for Research and Development, vol. 13, pp.226-296, May 1969). It was found that the mismatch of thermal expansioncoefficients between the semiconductor chip (usually silicon) and thesubstrate (usually ceramic or laminated plastic) causes strain andresultant stress in the coupling members (usually solder balls) andtheir joints. In the subsequent so-called C-4 technology, IBM reducedthese stresses by placing polymeric material in the gap between chip,substrate and solder balls and completely filling this gap.

Equipment for applying this underfill material, and processes forpolymerizing and “curing” this material have been described, forexample, in U.S. Pat. No. 6,213,347, issued Apr. 10, 2001 and U.S. Pat.No. 6,228,680, issued May 8, 2001 (Thomas, “Low Stress Method andApparatus for Underfilling Flip-chip Electronic Devices”). A variety ofunderfill dispensing techniques is reviewed in “Emerging Trends DriveEvolution of Underfill Dispensing” (S. J. Adamson, W. Walters, D. L.Gibson, and S. Q. Ness, Advanced Packaging vol. 9, no. 6, June 2000).

The problem of reliable adhesion of the underfill material to thesemiconductor chip and to the various substrates employed has not beenaddressed in known technology. On the other hand, in the late 1980's andearly 1990's, efforts had been undertaken by the semiconductor industryas well as by the National Institute for Standards and Technology, toimprove the quality and reliability of wire bonding. The challenge wasto clean the aluminum bond pads of silicon integrated circuit (IC) chipsfrom residues of photoresist, which was left on the pads from the priorwindow opening step. The gold ball of the wires was then able to formuniform gold/aluminum intermetallics. The cleaning method investigatedconsisted of an exposure of the silicon wafer to a plasma capable ofsputtering away the photoresist residues (“ashing”). A related techniqueis discussed in U.S. Pat. No. 5,731,243, issued Mar. 24, 1998 (Peng etal., “Method of Cleaning Residue on a Semiconductor Wafer Bonding Pad”).

In known technology, no effort has been undertaken to investigatesimilar cleaning methods to non-metallic surfaces, especially with thegoal of improving the contact quality to metallic or non-metallicmaterials. An urgent need has, therefore, arisen for a coherent,low-cost method of enhancing adhesion between semiconductor-to-polymericand polymeric-to-polymeric surfaces. When applied to semiconductordevices, the method should provide excellent electrical performance,mechanical stability and high reliability. The fabrication method shouldbe simple, yet flexible enough for different semiconductor productfamilies and a wide spectrum of design and process variations.Preferably, these innovations should be accomplished without extendingproduction cycle time, and using the installed equipment, so that noinvestment in new manufacturing machines is needed.

SUMMARY OF THE INVENTION

The present invention describes a plasma conditioning method ofimproving the adhesion between an integrated circuit chip, having activeand passive surfaces, the active surface polymer-coated and having aplurality of electrical coupling members, and an insulating underfillmaterial. The method comprises the steps of positioning a wafer having aplurality of integrated circuits, including the coupling members, in avacuum chamber of a plasma apparatus so that the polymer-coated surfacefaces the plasma source. Next, a plasma is initiated; the ion mean freepath is controlled so that the ions reach the wafer surface withpredetermined energy. The wafer surface is then exposed to the plasmafor a length of time sufficient to roughen the polymer surface, cleanthe polymer surface from organic contamination and improve the surfaceaffinity to adhesion. The adhesion ability of this surface to organicunderfill material is thus enhanced.

Acceptable adhesion improvement is also obtained when the plasmaconditioning process is performed on wafers prior to attaching thecoupling members to the ICs. In this case, the roughening of thepolymer-coated surface is slightly less.

In the first embodiment, the polymer coat consists of polyimide; in thesecond embodiment, the coat is poly-benzoxasole. The plasma may beformed in an oxygen/argon mixture, or in an oxygen/nitrogen mixture.

The effect of the plasma conditioning results in a mechanical rougheningof the polymer coat surface, as clearly visible in microscopicexamination; further in a cleaning of that surface, especially fromorganic contamination, as clearly measured by the contact angle of awater drop on the surface. In addition, the polymer coat surfacechemistry is modified and the surface energy increased. Together, thesecombined effects provide an improved adhesion capability of the polymercoat surface for underfill material, which is employed after flip-chipassembly of the semiconductor chip.

The improved adhesion between polymer-coated semiconductor chip andunderfill material is demonstrated by a test, in which a force isapplied the device from the outside in an effort to break theflip-assembled chip from its substrate. As the test results show, in theplasma-conditioned devices it is the substrate which breaks rather thanthe interface between the polymer-coated chip and the underfill, or theinterface between the underfill and the substrate.

The technical advances represented by the invention, as well as theaspects thereof, will become apparent from the following description ofthe preferred embodiments of the invention, when considered inconjunction with the accompanying drawings and the novel features setforth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of a ball grid array typesemiconductor device having a flip-chip assembled chip and an underfillmaterial, illustrating the field of the present invention.

FIGS. 2A and 2B are schematic cross sections of a portion of the active,polymer-coated chip surface, with a coupling member attached, before theprocess of plasma conditioning according to the invention.

FIG. 2A depicts a screen printed solder bump.

FIG. 2B depicts a plated solder bump over plated copper bump.

FIG. 3 is a schematic top view of an IC chip in order to indicate thebump layout with varying bump densities.

FIGS. 4A and 4B are microphotographs of a portion of the polymer coatover the active chip surface.

FIG. 4A indicates the polymer surface before the process of plasmaconditioning.

FIG. 4B indicates the polymer surface after the process of plasmaconditioning.

FIGS. 5A, 5B and 5C are microphotographs of portions of the polymer coatover the active chip surface after the process of plasma conditioning.The photographs are taken at various locations and various solder bumpdensities of the chip surface.

FIG. 5A is a microphotograph taken at medium bump density, position A inFIG. 3.

FIG. 5B is a microphotograph taken at high bump density, position B inFIG. 3.

FIG. 5C is a microphotograph taken at low bump density, position C inFIG. 3.

FIGS. 6A, 6B and 6C are microphotographs of portions of the polymer coatover the active chip surface after the process of plasma conditioning.The photographs are taken after various times of plasma conditioning.

FIG. 6A is a microphotograph taken after 12 min of plasma conditioning.

FIG. 6B is a microphotograph taken after 14 min of plasma conditioning.

FIG. 6C is a microphotograph taken after 16 min of plasma conditioning.

FIGS. 7A and 7B describe the structure of negative photosensitivepolyimide in the first embodiment of the polymer coat over the activechip surface.

FIG. 7A applies for the ester type of the polyimide.

FIG. 7B applies for the ionic type of the polyimide.

FIG. 8 describes the formation of poly-benzoxasol (PBO) in the secondembodiment of the polymer coat over the active chip surface.

FIGS. 9 to 11 are schematic cross sections of various plasma apparatussuitable for the plasma conditioning of semiconductor wafers accordingto the invention.

FIG. 9 refers to a reactive ion etch asher.

FIG. 10 refers an inductively coupled plasma asher.

FIGS. 11A and 11B refer to a barrel asher.

FIG. 11A is a cross section perpendicular to the barrel axis, allowing atop view of a wafer.

FIG. 11B is a cross section along the barrel axis, allowing a crosssection of the wafers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is related to U.S. Pat. No. 6,213,347, issued Apr.10, 2001 (Thomas, “Low Stress Method and Apparatus of UnderfillingFlip-chip Electronic Devices”), and U.S. Pat. No. 6,228,680, issued May8, 2001 (Thomas, “Low Stress Method and Apparatus for UnderfillingFlip-chip Electronic Devices”).

FIG. 1 is an example for the semiconductor device types, for which thepresent invention is extremely useful. FIG. 1 is a schematic crosssection of a ball grid array type semiconductor device, generallydesignated 100. The semiconductor chip 101 has an active surface 101 aincluding the integrated circuit, and a passive surface 101 b. Thesemiconductor chip 101 may be made of silicon, silicon germanium,gallium arsenide, or any other semiconductor material used in electronicdevice production.

Active surface 101 a is covered with a polymer coat 102. This polymercoat may be made of polyimide (PIQ) or poly-benzoxasole (PBO) ispreferably in the thickness range from about 2 to 8 μm. PIQ and PBOformulations are commercially available from Dow Corning, USA.

The active surface 101 a further has a plurality of electrical couplingmembers 103. These coupling members may be solder bumps selected from agroup consisting of tin/silver, indium, tin/indium, tin/bismuth,tin/lead, conductive adhesives, and solder-coated spheres. Preferably,they have a diameter from about 50 to 200 μm. The solder bumps may havevarious shapes, such as semispherical, half-dome, or truncated cone; theexact shape is a function of the deposition and reflow techniques andmaterial composition.

In other devices, these coupling members may be bumps selected from agroup consisting of gold, copper, copper alloy, or layeredcopper/nickel/palladium in the diameter range from 10 to 100 μm.Alternatively, the coupling members may consist of z-axis conductiveepoxy. The bumps may have various shapes, for instance rectangular,square, round, or half-dome.

As examples of coupling members, FIGS. 2A and 2B illustrate twovarieties of eutectic tin/lead solder bumps after solder reflow. As canbe seen from the schematic cross section in FIG. 2A, which refers to ICcopper metallization, the final interconnection copper layer 201 iscovered by the protective overcoat 202 (typically silicon nitride), inwhich a window has been opened. A cover layer 203, which ismetallurgically affine to the under-bump-metallization 204 and alsoadheres to copper, is positioned over copper layer 201. The polymer coat205, in turn, is positioned over the cover layer 203 and has a windowopened to allow contact with the under-bump-metallization 204. Arelative thick copper bump 206 is plated before the deposition of thesolder bump 207.

As can be seen from the schematic cross section in FIG. 2B, which refersto IC aluminum metallization, the final interconnection aluminum layer211 is covered by the protective overcoat 212 (typically siliconnitride), in which a window has been opened. The polymer coat 215 ispositioned over the protective overcoat layer 212; it has a windownested with the overcoat window. The under-bump-metallization 214 iscontacting the aluminum through this window. The deposited eutecticsolder is shown as bump 217 after reflow.

Referring now to FIG. 1. While the plasma conditioning process,described in more detail below, can be performed with good results onthe polymer coat 102 before the coupling members 103 are attached, it isthe preferred embodiment of the invention to perform the process afterattaching the coupling members 103. In the latter case, the density ofthe bumps influence the results qualitatively and quantitatively. Forthe case that the coupling members are solder bumps, the preferredresults are obtained with a center-to-center spacing of the solder bumpsbetween about 100 and 500 μm.

In many IC chips, the coupling member density varies significantlyacross the surface of the chip. An example is shown by the top view ofFIG. 3. It depicts the actual solder bump distribution of a specific ICdevice. The average bump density in area 1 is medium, in area 2 high,and in area 3 low.

The plasma conditioning process described below is performed in waferform. After completion of the plasma step, the chips are singulated fromthe wafer and assembled to the substrate. In FIG. 1, chip 101 and itscoupling members 103 are depicted assembled, face down (flip-chipprocess), onto a two-metal-layer substrate 110. It should be pointedout, however, that the number of metal layers may vary widely, from 1 to10 and more. Substrate 110 is made of electrically insulating materialssuch as polyimide, preferably in the thickness range from about 40 to 80μm; in some instances, it may be thicker. Other suitable materialsinclude Kapton™, Upilex™, PCB resin, FR-4 (which is an epoxy resin), ora cyanate ester resin (sometimes reinforced with a woven glass cloth).These materials are commercially available from several sources; asexamples, in the U.S.A., companies include 3-M, DuPont, and Sheldahl; inJapan, Shinko, Shindo, Sumitomo, and Mitsui, and Ube Industries Ltd; andin Hong Kong, Compass.

In the face-down assembly process, the coupling members 103 form a gapbetween the polymer coat 102 and the substrate 110. After chip assembly,polymeric underfill material 120 is used to fill this gap. A preferredtechnique for apparatus and method for underfilling is described in U.S.Pat. No. 6,213,347, issued Apr. 10, 2001, and U.S. Pat. No. 6,228,680,issued May 8, 2001 (Thomas, “Low Stress Method and Apparatus forUnderfilling Flip-chip Electronic Devices”). The preferred material forunderfilling is an epoxy filled with boron nitride or with aluminumnitride; the epoxy is a bisphenol A with an anhydride cross-linkingagent. Epoxy formulations are commercially available from Dexter, USA.

These underfill materials 120 adhere to both the polymer coat 102 andthe substrate 110. After completing the plasma conditioning for polymercoat 102, the adhesion of underfill 120 to coat 102 and to substrate 110is so strong that a force applied from outside breaks the substrate 110,rather than the interface between the underfill 120 and the polymer-coat102, or the interface between the underfill 120 and the substrate 110.

The device in FIG. 1 further has encapsulation material 130, preferablyapplied by a transfer molding process. Encapsulation 130 protects thepassive surface lolb of chip 101, and at least a portion 111 ofsubstrate 110 not covered by the attached chip 101. Encapsulationmaterial 130 may be polymerizable epoxy and thus endow stability andeven rigidity to device 100. Embedded in encapsulation 130 may be a heatspreader 131.

In FIG. 1, a plurality of solder balls 140 are attached to substrate 110opposite to the attached chip 101 and the encapsulation material 130.Solder balls 140 serve as ball grid array connections to printed wiringboards or electrical parts.

The plasma conditioning process of this invention is performed in waferform, with the active wafer surface coated by the PIQ layer, oralternatively by the PBO layer. In the preferred embodiment, theplurality of electrical coupling members are attached to the activewafer surface before starting the plasma process. Alternatively, thecoupling members are attached after the plasma conditioning. The resultsof the plasma conditioning are:

-   To roughen the PIQ, or PBO, surface;-   to clean the PIQ, or PBO, surface from unwanted organic and    inorganic contamination; and-   to improve the polymer surface affinity to adhesion. All of these    components contribute to the enhancement of the adhesion ability of    the PIQ, or PBO, surface to the epoxy-based underfill material.    PIQ or PBO Surface Roughening

The most significant contribution to adhesion improvement is derivedfrom the mechanical roughening of the PIQ, or PBO, surface. Themicrophotographs of FIG. 4 explain the reason. FIG. 4A is a photographunder 60,000× magnification of the PIQ surface before plasmaconditioning. As can be seen, the surface appears smooth on that scale.Consequently, the adhesion of this PIQ surface to the underfill materialis poor.

In contrast, FIG. 4B is a microphotograph under 60,000× magnification ofthe PIQ surface after plasma conditioning. As can be seen, the surfaceis rough. The peak-to-valley texture of this and followingmicrophotographs is about 0.1 to 3% of the PIQ layer thickness.Consequently, the adhesion of this PIQ surface the underfill material isexcellent.

FIGS. 5A to 5C illustrate the effect of the solder bump density on thesurface roughness of the PIQ surface achieved by plasma conditioning. Inall photographs, the magnification of the PIQ surface is 60,000×. FIG.5A demonstrates the PIQ after-plasma surface roughness in the proximityof a medium solder bump density (region 1 in FIG. 3). FIG. 5B shows thePIQ after-plasma surface roughness in the proximity of a high solderbump density (region 2 in FIG. 3). FIG. 5C shows the PIQ after-plasmasurface roughness in the proximity of a low solder bump density (region3 in FIG. 3). Although in every case significant surface texture can beachieved by the plasma, resulting in greatly improved adhesion to theunderfill material, FIG. 5 clearly demonstrate the favorable influenceof a high solder bump density for pronounced after-plasma surfaceroughness.

Auger analysis of the plasma-conditioned PIQ or PBO surface hasidentified numerous tin or lead depositions in the neighborhood oftin/lead solder bumps. These depositions protect the PIQ or PBO materialunderneath from further plasma bombardment so that steep elevations ofPIQ or PBO are created in a landscape eroded by the continued plasmabombardment. FIG. 6A illustrates an example of the resulting roughsurface contour after 12 min plasma exposure in a bump-near area(60,000× magnification).

How steeply the continued plasma exposure may contour the polymersurface, when a high density of tin/lead depositions has been createdaround the bumps in the early phase of the bombardment, is depicted inthe example of FIG. 6B (14 min plasma treatment). It is obvious thatsurfaces as rough as shown in FIG. 6B are especially suitable for strongadhesion to subsequent epoxy-based materials.

FIG. 6C shows another example of a rough surface created by tin/leaddepositions of less concentration (plasma exposure 16 min). FIG. 6Cresembles FIG. 5B. While adhesion will be extra strong in theseexamples, it should be repeated that even less pronounced surfacecontours like in FIG. 5C create favorable conditions for significantlyimproved coat-to-underfill adhesion.

PIQ or PBO Surface Cleaning

As mentioned above, photoresist residues, left over from process stepsof opening contact windows, have caused problems in the late 1980's forcreating reliable, reproducible gold-to-aluminum wire bonds. Theexperience gained in identifying these patchy, mechanically toughdeposits by Auger analysis, can be employed to identify any photoresistleft-overs from the window-opening process steps described in FIGS. 2Aand 2B. The plasma parameters used for PIQ or PBO conditioning can thenbe adjusted to remove these contaminant reliably; see below for plasmaprocess description.

A simple yet sensitive test for polymer surface cleanliness is theso-called water drop test. For clean surfaces, the contact angle of awater drop is significantly less than 10°. In contrast, film- orpatch-contaminated surfaces often show contact angles of 30° or more.

PIQ or PBO Surface Affinity To Adhesion

FIGS. 7A and 7B describe the structure of negative photosensitivepolyimide in the first embodiment of the polymer coat over the activechip surface. FIG. 7A, applicable to the ester type of polyimide, showsthe photoreactive group indicated by “P”. Enhancing the number of“dangling bonds” and adhesion sites by the plasma conditioningcontributes to the improved surface affinity to adhesion.

FIG. 7B, applicable to the ionic type of polyimide, shows the electricalcharges involved in the locations of ionic binding. Here again, “P”Iindicates the photoreactive group. Enhancing the number of “danglingbonds” and adhesion sites by the plasma conditioning contributes to theimproved surface affinity to adhesion.

FIG. 8 describes the formation of poly-benzoxasol in the secondembodiment of the polymer coat over the active chip surface. The formulashows the base polymer as the precursor of the CRC-8000 series. Heattreatment is then transforming the base polymer into poly-benzoxasole asthe post-bake formulation of the polymer.

FIGS. 9 to 11 are schematic cross sections of a number of differentplasma apparatus, and thus plasma processes, which are all suitable forthe plasma conditioning of whole semiconductor wafers according to thepresent invention. It should be stressed that the plasma conditioningmethod of the present invention is equally successful for whole wafersbefore “sawing” into individual chips, or after singulation intoindividual chips. In the latter case, the plurality of chips remainsstill attached to the supporting tape (the so-called “blue tape” heldwithin a sturdy frame).

The apparatus of FIG. 9 is referred to as a “reactive ion etch asher”.In FIG. 9, inside the bell jar 901 are the pedestal 902 for the wholewafer 905 and the electrode 903. Pedestal 902 is a water cooled baseplate, allowing temperature control of the wafer during plasmaconditioning. Arrows 904 indicate the gas flow. The chamber size, givenby bell jar 901, is typically 30 cm diameter and between 10 and 25 cmtall. The distance between the wafer 905 and the electrode 903 can varyfrom about 2 to 12 cm. The bias applied between wafer and electrode mayvary from 300 to 450 V.

The plasma comprises a mixture of oxygen and argon, or of oxygen andnitrogen. For conditioning PIQ surfaces, the plasma is preferablycontrolled to a flow of 2000 to 3000 sccm (standard cubic centimeter)oxygen and 700 to 1200 sccm argon at a pressure of 2 to 3 Torr. The timeof plasma exposure is preferably controlled to a period between 150 and250 s, but longer time periods have also been employed successfully.

For conditioning PBO surfaces, the plasma is preferably controlled to aflow of 2000 to 3000 sccm oxygen and 800 to 1100 sccm argon at apressure of 2 to 3 Torr. The time of plasma exposure is preferablycontrolled to a period between 130 and 200 s, but longer time periodshave also been employed successfully.

The apparatus of FIG. 10 is referred to as a “inductively coupled plasmaasher”. In FIG. 10, inside the bell jar 1001 are one or more pedestals1002 for the whole wafers 1005 and the plasma tubes 1003. Pedestals 1002are cooled base plates, allowing temperature control of the wafers (forinstance, to 60° C.) during plasma conditioning. The chamber size, givenby bell jar 1001, is typically 90 by 40 cm, and between 10 to 15 cmtall. The plasma quartz tubes 1003 above chamber 1001 are typically 22cm diameter and 12 cm long.

The apparatus of FIGS. 11A and 11B is referred to as a “barrel asher”.In FIGS. 11A and 11B, inside the tube 1201 is a boat 1202 for aplurality of wafers 1205. Arrows 1204 indicate the gas flow. The chambersize, given by tube 1201, is typically 30 to 35 cm diameter and 30 to 35cm length. Boat 1202 may contain 1 to 10 or more wafers. The plasmaconditions are as described above.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

1. A semiconductor device, comprising: an integrated circuit chip havingactive and passive surfaces, said active surface polymer-coated with apolymer having enhanced dangling bonds; said chip assembled face-downonto a substrate, and polymeric underfill material filling a gap,between the polymer and the substrate, said underfill material adheringto said polymer-coat and said substrate; said adhesion of said underfillmaterial being so strong that a force applied from outside breaks saidsubstrate rather than the interface between said underfill and saidpolymer-coat or said substrate.
 2. The device of claim 1 furtherincluding a plurality of electrical coupling members, said couplingmembers forming a gap between said polymer coat and said substrate. 3.The device according to claim 1 further comprising: encapsulationmaterial protecting said passive chip surface and at least a portion ofsaid substrate not covered by said attached chip; and solder ballsattached to said substrate opposite to the attached chip andencapsulation material.
 4. The device according to claim 1 wherein saidpolymer coat consists of at least one of polyimide (PIQ) orpoly-benzoxasole (PBO) in the thickness range from about 2 to 8 μm. 5.The device according to claim 1 wherein said underfill material is atleast one of an epoxy filled with boron nitride or with aluminumnitride; the epoxy being a bisphenol A with an anhydride cross-linkingagent.
 6. The device according to claim 1 wherein said substrate is anorganic material and is selected from a group consisting of polyimidefilm, FR-4, FR-5 and BT resin.